The present invention relates to a technology in pedestal level control circuits for video signal processing in a television receiver, etc., more precisely, for pedestal level adjustment, such as brightness adjustment, cutoff adjustment for adjusting white balance of color video images.
In a conventional television receiver, the pedestal level of a video signal output from a tuner or the like is not always constant. Therefore, for brightness adjustment, the pedestal level is clamped to a predetermined level and then is offset by a predetermined amount.
Furthermore, when a plurality of video signals are switched for use, for example, when an external video signal is input from external equipment, the adjustment as described above is performed with respect to these video signals to equalize the pedestal levels between them.
For display of color video images, cutoff adjustment in which the pedestal level is adjusted in the above-described manner for video signals of each of, for example, R, G, and B (red, green, and blue) is performed in order to adjust white balance in accordance with non-uniformity in the emission of a cathode ray tube (CRT).
Hereinafter, a conventional pedestal level control circuit for performing the above-described brightness adjustment or cutoff adjustment will be described more specifically.
FIG. 9 is a block diagram showing a conventional pedestal level control circuit (brightness adjustment circuit) for adjusting brightness of both internal and external video signals. In FIG. 9, a clamp circuit 501 fixes the pedestal level of an internal video signal while being input from, for example, a tuner, to a predetermined level. A clamp circuit 502 fixes the pedestal level of an external video signal while being input from external equipment, to the same level as the internal video signal. A reference voltage source 503 sets the predetermined pedestal level. A switch 504 selects internal video signals or external video signals. An amplifier 505 amplifies an input video signal, and converts, through a load resistor 505b, a current while being output from a transistor 505a into a voltage that the amplifier 505 outputs. A D/A converter 506 converts a digital adjustment data signal that is for brightness adjustment into an analog voltage signal. A brightness adjustment current output circuit 507 converts the analog voltage signal to a current signal and supplies it to the amplifier 505 so that the pedestal level of the video signal output from the amplifier 505 is set to a level in accordance with the brightness adjustment data.
In the pedestal level control circuit having the above-described configuration, the clamp circuit 501 and the clamp circuit 502 both use the same reference voltage source 503, so that the pedestal levels of the internal and external video signals are clamped to the same level to eliminate fluctuations in the pedestal level. Moreover, the brightness adjustment is performed by offsetting the pedestal level in accordance with the brightness adjustment data, based on the pedestal levels of the same level, so that there is no difference in the basic luminance of video images to be displayed, regardless of which the switch 504 is directed to.
FIG. 10 shows another conventional pedestal level control circuit (brightness and cutoff adjustment circuit) which has a configuration for performing brightness adjustment and cutoff adjustment with respect to internal and external video signals of three channels of RGB. In FIG. 10, both the reference voltage source 503 and the D/A converter 506 are the same as those shown in FIG. 9. The clamp circuit 501, the clamp circuit 502, the switch 504, and the amplifier 505, each being respectively the same as in FIG. 9 are provided for each of the R, G, and B video-signal channels. However, in addition to a signal current for brightness adjustment, respective signal currents for cutoff adjustment are input to each amplifier 505. The brightness adjustment current output circuit 507 outputs the signal current for brightness adjustment to the amplifiers 505 for R, G and B. Furthermore, a D/A converter 508 and a cutoff adjustment current output circuit 509 are provided. The D/A converter 508 converts a digital data signal for cutoff adjustment for each of R, G, and B to an analog voltage signal, and the cutoff adjustment current output circuit 509 converts the analog voltage signal to a current signal and supplies it to the amplifier 505 so that the pedestal level of the video signal output from each amplifier 505 is set to a level in accordance with the brightness adjustment data and the cutoff adjustment data.
In the pedestal level control circuit having the above-described configuration, as in the circuit shown in FIG. 9, the pedestal levels for all the R, G, and B and the internal and external video signals are clamped to the same level. Therefore, regarding brightness adjustment, the pedestal levels are offset by an amount in accordance with the brightness adjustment data for all the cases. On the other hand, regarding the cutoff adjustment, the pedestal level is offset by an amount in accordance with the cutoff adjustment data of each corresponding color. Therefore, there is no difference in the luminance of video images to be displayed, regardless of the switching of the switch 504 either to an internal video signal or an external video signal, and white balance suitable for non-uniformity in the emission of a CRT or the like can be obtained.
However, in the conventional pedestal level control circuits, the clamp circuit is provided for each video signal, so that the circuit scale tends to be large. Furthermore, it is necessary to perform brightness adjustment or cutoff adjustment after clamping the video signal. Therefore, for example, configuring elements or blocks subsequent to the clamp circuit in one IC chip requires input terminals (pins) to which the brightness adjustment signal or the like are input; digital input interfaces; power source circuits and power source terminals of two types for digital and analog signals; and lines between the IC chips (bus lines) or the like. Moreover, in a case in which the brightness adjustment or the like is performed based on digital signal processing and the D/A converter or the like is provided in the vicinity of the clamp circuit or the like, video signals are more susceptible to noise caused by the digital signals.